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Macquarie Island is an exposure above sea-level of part of the crest of the Macquarie Ridge. The ridge marks the Australia–Pacific plate boundary south of New Zealand, where the plate boundary has evolved progressively since Eocene times from an oceanic spreading system into a system of long transform faults linked by short spreading segments, and currently into a right-lateral strike-slip plate boundary. The rocks of Macquarie Island were formed during spreading at this plate boundary in Miocene times, and include intrusive rocks (mantle and cumulate peridotites, gabbros, sheeted dolerite dyke complexes), volcanic rocks (N- to E-MORB pillow lavas, picrites, breccias, hyaloclastites), and associated sediments. A set of Macquarie Island basaltic glasses has been analysed by electron microprobe for major elements, S, Cl and F; by Fourier transform infrared spectroscopy for H  2  O; by laser ablation–inductively coupled plasma mass spectrometry for trace elements; and by secondary ion mass spectrometry for Sr, Nd and Pb isotopes. An outstanding compositional feature of the data set (47·4–51·1 wt % SiO  2  , 5·65–8·75 wt % MgO) is the broad range of K  2  O (0·1–1·8 wt %) and the strong positive covariation of K  2  O with other incompatible minor and trace elements (e.g. TiO  2   0·97–2·1%; Na  2  O 2·4–4·3%; P  2  O  5   0·08–0·7%; H  2  O 0·25–1·5%; La 4·3–46·6 ppm). The extent of enrichment in incompatible elements in glasses correlates positively with isotopic ratios of Sr (  87  Sr/  86  Sr = 0·70255–0·70275) and Pb (  206  Pb/  204  Pb = 18·951–19·493;   207  Pb/  204  Pb = 15·528–15·589;   208  Pb/  204  Pb = 38·523–38·979), and negatively with Nd (  143  Nd/  144  Nd = 0·51310–0·51304). Macquarie Island basaltic glasses are divided into two compositional groups according to their mg-number–K  2  O relationships. Near-primitive basaltic glasses (Group I) have the highest mg-number (63–69), and high Al  2  O  3   and CaO contents at a given K  2  O content, and carry microphenocrysts of primitive olivine (Fo  86–89·5  ). Their bulk compositions are used to calculate primary melt compositions in equilibrium with the most magnesian Macquarie Island olivines (Fo  90·5  ). Fractionated, Group II, basaltic glasses are saturated with olivine + plagioclase ± clinopyroxene, and have lower mg-number (57–67), and relatively low Al  2  O  3   and CaO contents. Group I glasses define a seriate variation within the compositional spectrum of MORB, and extend the compositional range from N-MORB compositions to enriched compositions that represent a new primitive enriched MORB end-member. Compared with N-MORB, this new end-member is characterized by relatively low contents of MgO, FeO, SiO  2   and CaO, coupled with high contents of Al  2  O  3  , TiO  2  , Na  2  O, P  2  O  5  , K  2  O and incompatible trace elements, and has the most radiogenic Sr and Pb regional isotope composition. These unusual melt compositions could have been generated by low-degree partial melting of an enriched mantle peridotite source, and were erupted without significant mixing with common N-MORB magmas. The mantle in the Macquarie Island region must have been enriched and heterogeneous on a very fine scale. We suggest that the mantle enrichment implicated in this study is more likely to be a regional signature that is shared by the Balleny Islands magmatism than directly related to the hypothetical Balleny plume itself.

Enriched End-member of Primitive MORB Melts: Petrology and Geochemistry of Glasses from Macquarie Island (SW Pacific)